Many electrical systems and equipment are characterized in use by the radiation therefrom of extraneous electromagnetic wave energy generated as an incident to the operation thereof. Such systems and equipment run the gamut from, say, power lines (60 cycle hum) to, say, microwave ovens (microwave energy leakage), and the frequencies of such radiated extraneous waves are distributed over a correspondingly broad spectrum. The presence in the environment of radiated energy of such sort is generally undesirable because it is a form of "noise," and because it may adversely affect the operation of other electrical or electronic systems and equipment in the vicinity of the source of its emanation. Accordingly, that kind of emitted electromagnetic wave energy is referred to as electromagnetic interference or, as abbreviated, "EMI."
With the advent of telecommunications and computing equipment which employ digital techniques and clock pulses having a high repetition frequency, the problem of EMI emission therefrom has become troublesome to the extent that the Federal Communications Commission has promulgated regulations providing that the EMI radiation from such equipment must not exceed certain prescribed maximum permissible levels. A further characteristic of such equipment is that it employs as plug-in components a large number of circuit cards and (as described in the article entitled "Modeling Electromagnetic Interference Properties of Printed Circuit Boards," by Clayton R. Paul, published on pages 33-50 of the Jan. 1989 issue of the "IBM Journal of Research and Development," the boards of those cards are rich sources of EMI. Consequently, there is a need for means to reduce the EMI emitted by this equipment and originated from the cards therein to the point where such interference is less by a safe margin than the maximum permissible level specified therefor by the FCC and, preferably, is far enough below that level to assure that it will not be exceeded in any foreseeable circumstance of practical use of the equipment.
In equipment of the kind considered which employs circuit cards as components thereof, the cards are commonly retained in the equipment in bays which are spaces between shelves included in the equipment. Each such bay is bounded at its top and bottom by upper and lower ones of such shelves, at its rear by a backplane, and at horizontally opposite ends of such rear, by vertical sidewalls extending from the rear to the front of the bay. Each such bay is, moreover, open at its front and has a front opening in the shape of a rectangle of greater horizontal then vertical dimension. The one or more bays of such equipment each normally contain a bank of circuit cards inserted into the bay from its front and retained in the bay such that the cards in the bank are vertical and in side-to-side stacked relation. When such cards are included in telecommunications equipment wherein each card serves a communications channel or channels respective thereto, such a bank is called a "channel bank." Here, however, such a bank will be referred to more generically as a card bank or circuit card bank.
When the described circuit card apparatus is operating, electromagnetic wave energy originally generated by circuit cards in a bay escapes as radiation from that bay into the exterior environment through the bay's rectangular front opening which is bordered around it periphery by a closed-loop, a.c. electrical circuit path. In those circumstances, the front opening acts as a slot antenna for such radiation. It is an empirical rule for such a slot antenna that it is a good radiator only of electromagnetic waves shorter in wavelength than the larger of the two dimensions of the rectangular opening, the slot antenna tending to suppress radiation of larger wavelength. It follows from such a rule that, if EMI escaping from the circuit cards into the bay is distributed fairly evenly over the frequency spectrum, a decrease in the size of such larger dimension will produce a decrease in the EMI "wattage" radiated out through such front opening to the exterior.
As another empirical rule, however, it is only a reducing of the larger of the two dimensions of the front opening which will be primarily effective in reducing the EMI power radiated through the opening. To put it another way, if the larger dimension is kept constant and the smaller dimension is reduced in size, the radiated EMI power will not be relatively reduced by anywhere near the amount as the reduction in radiated EMI power effected by reducing the larger opening dimension. Consequently, an aperture at the front of the bay which is narrow but of the full length of the bay (as, say, an interstitial gap between the housing surrounding the front of the bay and the edge of a cover closure for the bay) will, despite its small width, permit an undesirably large amount of EMI to escape from the bay to the exterior.
U.S. Pat. No. 4,762,966, issued Aug. 9, 1988, in the name of David E. Kosanda, to Rockwell International Corporation, proposes to reduce EMI escaping from a space within a box for containing circuit cards by providing a cover for the front of such space so that it is closed at top and bottom except for narrow interstitial gaps left between the top and bottom edges of the cover and adjacent portions of the box, and by providing for such gaps what is referred to in the patent as an "EMI gasket." That gasket comprises for each such gap a plurality of resilient metallic fingers on the cover and disposed on its edge on the outer side of such gap to be spaced from each other in the gap's length and to project forwardly from the cover towards the portion of the box which is on the gap's inner side. When the cover is attached to the box (in some way not disclosed by Kosanda), the forward ends of the fingers are described by the patent as being forced back towards the edge of the cover or panel, during which time electrical contact is maintained between the cover and the box, and the spacing between the various fingers for the top and bottom gaps "is such that signal frequencies of lower than a given frequency cannot pass through the opening."
The EMI reducing scheme proposed by Kosanda (to the extent it is understood from the disclosure of that patent) has, however, the disadvantages, among others, that the cover contemplated thereby is an additional item of expense for the circuit card containing apparatus, hides from view the faceplates of the circuit cards inserted in the box (and thus precludes any casual visual inspection of those faceplates and any associated indicating devices), must be opened or removed each time a card is to be added to or removed from the bank thereof in the box, and takes up space at the front of the box.